1. Field of the Invention
This invention relates to improved electrophoretic dispersions and a method for improving the performance of an electrophoretic display.
2. Brief Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a dielectric solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other and separated by spacers. One of the electrodes is usually transparent. A suspension composed of a dielectric solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of particles such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric solvent and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology was disclosed in U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO01/67170), now U.S. Pat. No. 6,930,818, U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (corresponding to WO02/01280), now U.S. Pat. No. 6,672,921, and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001 (corresponding to WO02/65215), now U.S. Pat. No. 6,933,098. The improved EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent or solvent mixture.
As in liquid crystal and other displays, an EPD may be a segment display, a passive matrix display or an active matrix display, depending on the driving mechanism and circuitry design. The passive matrix driving system is one of the most cost effective driving mechanisms. The system has row electrodes on the top side and column electrodes on the bottom side, of the cells. In most cases, the top row electrodes and the bottom column electrodes are perpendicular to each other. Generally, a threshold voltage of at least ⅓ of the driving voltage is required to suppress or eliminate the undesirable crosstalk or cross-bias effect in adjacent pixels of a passive matrix display.
Crosstalk occurs when the particles in a cell are biased by the electric field of a neighboring cell. Widening the distance between adjacent cells may eliminate such a problem; but the distance may also reduce the resolution of the display.
Alternatively, the crosstalk problem can be lessened if a cell has a significantly high threshold voltage. A large gamma (or a steep slope) of the response-voltage characteristic curve is also desirable to increase the resolution of a passive matrix device. However, cells in EPDs formed using the electrophoretic materials and techniques currently available typically do not have the required response-voltage characteristics to prevent the undesirable movement of particles. As a result, the EPDs constructed from these materials and techniques usually cannot achieve high resolution.
Cross bias is another well-known problem associated with a passive matrix display. The voltage applied to a column electrode not only provides the driving bias for the cells in the scanning row, but it also affects the bias across the non-scanning cells in the same column. This undesired bias may force the particles of non-scanning cells to migrate to the opposite electrode. This undesirable particle migration causes visible optical density change and reduces the contrast ratio of the display.
In addition, in order to scan through all rows of electrodes in a frame within a reasonable time scale, a fast response is also highly desirable. However, none of the EPDs currently available has shown an acceptable threshold characteristics or response speed required.
Most electrophoretic dispersions do not have the required threshold characteristics to suppress or eliminate the undesirable cross-talk or cross-bias among adjacent pixels during matrix driving. Electrophoretic dispersions with a threshold characteristics have been reported by, for example, I. Ota, et al, in SID Proceedings, 18, 243 (1977) and Evans, et al, in U.S. Pat. No. 3,612,758. In most cases, the threshold voltage was achieved with trade-offs in, for example, response time, operating voltage, image uniformity or display longevity, probably due to irreversible flocculation and/or network formation, and sometimes, undesirable redox reaction(s) and/or electrodeposition at the electrode surface.
To suppress the cross effect, an additional conductor layer or grid electrode has been disclosed in, for example, IEEE Trans. Electr. Dev., p. 827, July (1977) and U.S. Pat. Nos. 3,612,758, 4,655,897, 5,177,476 and 5,460,688, U.S. Ser. No. 10/242,335, filed on Sep. 11, 2002 (corresponding to WO 03/023510), now U.S. Pat. No. 6,781,745 and U.S. Ser. No. 10/282,444, filed Oct. 28, 2002 (corresponding to WO 03/038512), now U.S. Pat. No. 6,806,495. However, the manufacturing cost for multilayer electrode structures is high. Alternatively, magnetic particles and a magnetic electrode have been disclosed in U.S. Pat. No. 6,239,896 (assigned to Canon) to provide the required threshold, but also at the expense of manufacturing cost.
Therefore, there is a need for a cost effective method for inducing or enhancing the threshold characteristics without the trade-offs in image uniformity and display longevity, and without the need of complex circuitry designs.
The content of each document referred to in this application is incorporated by reference into this application in its entirety.